Process for phosphating stainless steel surfaces



Patented July 19, 1949 PRBCESS FOB PHOSPHATING STAINLESS STEEL SURFACES Arthur Zavarella, Agawani, Mass.

No Drawing. Application September 3, 1946, Serial No. 694,439

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3'70 0. G. 757) 3 Claims.

The invention described herein may be manrufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to improvements in the treatment of metallic pieces and is concerned more particularly with the treating of stainless steel.

It is a particular object of the invention to provide an improved stainless steel which is especially adapted for use in the manufacture of firearms. That is to say, the novel characteristics of the invention are peculiarly adapted to provide a so-called weapon steel which has an exterior surface which is substantially free from, shine or luster and is also non-gal-ling. At the same time the resultant exterior surface is both wearresistant and capable of being painted as desired.

In other words, it is important to eliminate any shine on the exposed exterior surfaces of ordnance weapons since government inspectors of ordnance materiel will reject steel having reflecting surfaces, but it is obviously desirable from a safety standpoint in the field during warfare that weapon steel have no shine.

In general, the method of the invention includes the following essential sequential steps:

1. A metallurgical step which consists of nitriding .or carbur-izing the piece or pieces in order to harden their exterior surfaces and more or less convert the outer skin of the steel to a condition where it has lost its relative good resistance to corrosion.

2. A mechanical step, which consists of gritblasting or shot-peening in order to homogenize the exposed surface. This consists :of a coldworking operation on the said exterior surface of the metal in order to produce a localized and uniform stress thereon.

3. A chemical step which is adapted to activate the exterior or exposed surface of the steel so that it is rendered more reactive to the subsequent step of immersion in a phosphating bath; and

4. The final step consisting of the immersion in a phosphate bath.

It has been found advisable, after the pieces have been nitrided, or carburized to uniformly stress and roughen the exposed surface of the steel in some manner, such as by so-called grit blasting or shot-:peening. pieces are subjected to a so-called cold-Working operation whereby their outer skins are caused to have a uniform or homogeneous surface and color.

When the exposed surfaces of the pieces are In other words the chemically activated by dipping the same in a solution of dilute phosphoric acid, most eflicient results have been achieved through such a dip which lasts approximately two minutes and by then rinsing them in ordinary water or some aqueous solution.

A stainless steel piece should only be hardened to a relatively slight depth so that its casing renders it vulnerable to corrosion in the processing baths. According to the process of this invention, while there is included a nitriding, carburizing, or other hardening step, there is in addition a chemical activating of the surface, subsequent to the hardening, for the deliberate purpose of rendering said outer skin or exterior surface corrodible.

This, in turn, is followed by immersion of the piece or pieces in a phosphate solution which results in a product which has been found to not only have a non-galling exterior surface but one which is durable, non-lustrous, and corrosion resistant and which may be easily and effectively covered if desired with olive-drab paint, for example, for special use on army equiment.

The phosphate coating preferably has a manganese base and consists of a combination of manganese phosphate and iron phosphate. Such a coating is more durable than coatings heretofore utilized and stands up better under sliding friction which, of course, is essential where the metal is used for the manufacture of ordnance weapons.

Preferably, the manganese-base phosphating bath contains an accelerator, such as nitric acid or the like. The conversion of the outer surface or so-called skin of the stainless steel, as above stated, renders it more susceptible to the subsequent steps previously referred to and the pieces or articles are then ready to obtain the maximum benefits from the phosphating bath so as to have a surface whic his resistant to abrasive Wear.

When the finished article, which has been treated according to this invention, is put into service, it is definitely resistant to abrasive wear and its exterior surface has been restored so as to have corrosion-resistant non-reflecting characteristics and also a uniform color.

I claim:

1. The method of treating a stainless steel piece of metal to resist abrasive wear for use in the manufacture of ordnance equipment consisting of, first metallurgically treating by surface impregnation with carbon or nitrogen to alter the exterior surface of said piece converting said exterior surface from stainless steel to a steel which has lost its relative resistance to corrosion, next mechanically treating by cold Working said exterior surface of said piece, then chemically activating said exterior surface by dipping in an acid solution of phosphoric acid, and finally subjecting said exterior surface to a manganese base phosphate bath to provide a non-lustrous, non-galling exterior surface which is resistant to abrasive wear.

2. The method of treating a stainless steel piece of metal to resist abrasive wear consisting of, first case hardening the exterior surface of said piece, next mechanically processing by cold working whereby said exterior surface is homogenized, uniformly stressed and roughened, then chemically activating said roughened surface with phosphoric acid, and finally subjecting said exterior surface to a manganese base phosphate bath to provide a non-lustrous, non-galling exterior surface which is resistant to abrasive wear.

3. The method of treating a stainless steel piece of metal to resist abrasive wear consisting of, first impregnating the exterior surface of said piece with carbon or nitrogen, next mechanically processing said exterior surface by shot-peening or REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,832,979 George Nov. 24, 1931 2,059,783 Farnworth Nov. 3, 1935 2,087,230 Bigeon July 20, 1937 2,131,710 Malcolm Sept. 27, 1938 2,172,422 Uhlig Sept. 12, 1933 OTHER REFERENCES Transactions of American Society for Steel Treating, vol. XXI 1933), pages 481 and 482. 

